Intensifying screens



United States Patent 3,231,342 INTENSIFYING SCREENS Alan S. Kollock, (Jentrai Bridge, N.Y., assignor to General Magnaplate Corporation, Belleville, NJ., a corporation of New Jersey No Drawing. Filed Mar. 5, 1963, Ser. No. 262,839 17 Claims. (Cl. 29183.5)

This invention relates to intensifying screens which are particularly adapted for use in film holders for strip film employed in industrial radiography, and as taught in my copending United States patent application Serial No. 84,687, filed January 24, 1961, now latent No. 3,119,015, and United States patent application Serial No. 129,143, filed August 3, 1961, now Patent No. 3,168,- 647, of which parent applications this is a continuationinpart.

Intensifying screens have been used for many years in radiographic inspection for the conversion of X or Gamma radiation to usable long wavelength electromagnetic energy and may be comprised of many different materials to suit different purposes. For example, those intensifying screens used for medical radiography are of a composition such as calcium tungstat-e, zinc sulfide, or barium platinocyanide which fiuoresce with a wave length in the visible light spectrum. All other materials fluoresce to a greater or lesser degree, most with an electromagnetic radiation outside of the visible range. It is known that metals emit characteristic radiation with their strongest emission in a wave length equvalent to the excitation potential of the K-alpha electrons, i.e.z

Excitation potential K-alpha (kev.)

Metal:

Bismuth 90 1 Tin 29.1 Lead 87.6 Tungsten 69.3

- of their ability to convert that radiation to heat and photoelectrons. This is assumedly a function of their atomic number. Therefore, elements of a higher atomic number have a greater number of electrons available which may be struck and converted to photographically usable energy. Hence, the emission of radiation has a chain effect. The radiation is converted to photoelectrons which have the ability to affect other atoms and generate radiation that has the ability to strike other atoms and generate photoelectrons, the losses in energy being dissipated as heat. Because photographic film employed in industrial radiography is most sensitive to radiation below 80 kv., in order to achieve the best image on the film and conversion of radiation to photoelectrons, the intensifying screen should have a characteristic radiation value less than that voltage.

intensifying screen material, for weld radiography and other curved applications should, of necessity, be flexible, capable of being bent, rolled into small diameters and otherwise deformed from a normal fiat sheetlike condition. One of the basic principles of radiography is that the film should be placed as close as possible to the part being radiographed in order to prevent geometric distortion. Another is that the intensifying screen should conform snugly to the film to eliminate or obviate the possible formation of air spaces between them that will absorb radiation and result in the creation of erroneous images on the film. Since X-ray, as light, sufi'er diminution in intensity in relation to the square of the distance it must travel, it follows that over relatively long film lengths and short focal film distances, the film should curve in order to display a uniform darkening along the length of the film.

Present-day intensifying screens are made of fairly rigid, thick and heavy metal foils from 0.005 inch and thicker and backing materials secured and combined together from relative movement in order to reduce the possibility of mechanical damage to the foil. This rigidity makes it extremely dilficult to conform the rigid, thick, heavy intensifying screen to small radii. Although the center of the intensifying screen may be curved or curled readily, the extremities tend to hold their flat shape and this causes the screen and backing combination to assume a parabolic shape. While this is particularly true of small radii curves, it is also noticed to a lesser degree on larger ones.

The ideal intensifying screen is one which can be curved to fit, if possible, about the whole inside diameter of a pipe to effect a substantial reduction in the number of radiographic exposures required to adequately inspect the pipe as compared with the use of present known rigid, thick and heavy intensifying screens. In addition, the ideal intensifying screen must be sufiiciently ductile to conform snugly to the film to be exposed, thus preventing the formation of the aforementioned radiation absorbing spaces between it and the film. Ductility of the intensifying screen is important because when it is curled or bent about small radii, the intensifying screen material and backing material each will seek to assume different radii and, therefore, bend unevenly. Hence, if the intensifying screen and backing material are sufficiently ductile, even after they are securely combined together into a single sheet, they will not move relative to each other and will not part or become unsecured from the other, thereby eliminating the possible formation of undesired wrinkles, bubbles, blisters and/or delaminations that would otherwise result in extraneous images created on the film.

-It is known that air, dust, chemical films, powders, and other dirt, absorb photoelectrons. Therefore, it follows that they must be excluded to the greatest possible extent from between the film and the electron emitting surface or intensifying screen. To obtain the maximum intensifying effect, the screen should be compressed to the surface of the film as tightly as possible. Unsuccessful attempts to achieve this by various mechanical media have been devised, such as evacuating air from between the screen and film. Further, the screen must be protected from mechanical damage, and must be rigid or have sufficient body to press tightly against the film in use.

The effect of mechanical damage follows two general patterns or conditions that result in a situation where the intensifying screen is removed or spaced from the film so it is not in intimate contact with the film, thereby resulting in an area of reduced intensity of the image, which, when the film is developed, will have a false image produced on it that may be confused with a defect in the radiographed article. One of these damages may be to the face of the screen by scratching or denting it. The second is caused by damage to the rear of the screen that result in raising areas on the face of the screen adjacent the film. These raised areas, whether wrinkles, pimples, bumps, creases or other defects, create a dark image on the developed film and force the surrounding areas of the intensifying screen to be separated or spaced from the film, resulting in a lighter image in those areas of the developed film.

The usual material from which intensifying screen foils are made, is 6% antimonial lead. The high percent of antimonial lead is used because it has a higher tensile strength than pure lead and is much harder. For some commercial applications in corrosive environments, it is much less susceptible to deterioration by attack than pure lead, tin, bronze and many other metals. However, it is attacked by the atmosphere and within a short time develops a coating of corrosion product. This corrosion product does not tightly adhere and dusts off, staining the film emulsion. Since the antimonial lead screen corrosion product is not as dense and homogeneous as is pure metal, it will not emit as many photoelectrons, but rather, absorbs them to the detriment of the radiographic image.

The phenomena of photoelectron emission tends to take place within the confines of individual crystals and in a sense causes the crystals to fluoresce in a manner similar to the tungstate screens used in medicine. The material from which the screens are made should, therefore, be as dense, fine grained, homogeneous and as uniform in thickness as possible, because large grains would reflect in the image as a mottling and resultant loss in definition of the screens.

The desideratum of this invention is to provide an intensifying screen that overcomes the aforementioned problems of presently known intensifying screens and that has the following desirable characteristics:

It is another object of this invention to provide an intensifying screen especially adapted to be used in film holders of a truly flexible nature such as disclosed in my aforementioned co-pending applications.

I have invented improved intensifying screens that are made from a special tin surfaced lead alloy in flat sheet form (hereinafter referred to as lead foil), which is in continuous secured contact with a backing material. The tin surfaced alloys used in preparing my intensifying screens contain, in addition to lead; antimony, between 2.7 and 3.4%, and preferably about 3%, and tin, between 0.075% and 3%. The percentages are based on the total weight of the lead foil although for a sheet of approximately 0.001 inch lead foil, the preferred tin composition is between 0.1% and 0.25%.

The tin present in the sheet of lead foil is in the form of a uniform and continuous surface coating that fully covers and shields the flat surfaces of the sheet. The tin coats the whole outer surface of the lead foil sheet that is adapted to be positioned adjacent to the emulsion of the film. The tin may also coat the inner surface of the foil, i.e. the surface in continuous secured contact with the backing material. Hence, a preferred embodiment of the intensifying screen invention comprises a flat lead foil sheet of approximately 0.001 inch thick having both of its flat surfaces coated with tin.

Extremely thin lead foil intensifying screens such as disclosed in my invention are susceptible to damage and must be protected from scratches and other defects that will create undesired extraneous images on the film. This is accomplished through the use of a backing material which, when secured and combined together with the thin sheet of lead foil, forms a laminate intensifying screen.

The flat thin sheet of lead foil intensifying screen, constructed according to my invention, is bonded or otherwise secured continuously throughout the length and width of one of its flat surfaces to a corresponding surface of the backing member thereby forming the laminate. This laminate is now capable of being employed as one of the combined film holder and intensifying screen sheets disclosed in my aforementioned co-pending applications.

The selection and characteristics of the protective backing material are important since normally any damage to the backing material will be transmitted to and reflected in the thin surface of the screen lead foil that will create a corresponding image on the developed film. The entire laminate combination of intensifying screen and backing material should have the flexibility but not stretchability of a sheet of rubber. Rubber itself is not a satisfactory material because its resilient, stretchable nature makes it very diflicult to maintain a uniformity or evenness over all its surface, resulting in a wavelik-e condition on the surface, which, of course, will be reflected in the radiographic image.

As a matter of fact, any irregularity in the surface of the backing material will be reflected through the lead foil intensifying screen and to the film. A mote of dust, a piece of dirt in the adhesive securing the lead foil to the backing, a gel spot, a striation from the extrusion die, all will be transferred to and appear on the face of the lead foil. Therefore, the surface of the backing material to which the lead foil is applied and even the bonding adhesive should have or approach optical flatness. Paper, it is known, is not optically flat but has a surface texture and roughness which is transmitted to the lead foil when they are laminated together. Most plastic materials, being extrusions, are relatively non-uniform in thickness, not optically flat, and are subject to striations from the dies, inclusions of foreign materials and hard spots (gel spots or fish eyes).

I have found that cellulose acetate or tri-acetate, and trademarked materials as Mylar or Cronar, manufactured by E. I. du Pont de Nemours Company, Wilmington, Delaware, are suitable backings because they are essentially optically flat and relatively strong. The backing material must be sufliciently strong although not necessarily thick so that it protects the lead foil from damage during handling. It should not be so thick that the flexibility of the intensifying screen is afliected. Thicknesses of Mylar and Cronar backing materials in the order of about 0.003 inch have proven strong and flexible and provide sufficient protection as backings for the 0.001 inch lead foils.

The backing material should also be opaque to actinic light. In those instances when the backing material is not sufiiciently opaque, an additional opaque layer may be added to the backing material on its surface remote from the secured or laminated lead foil. An excellent additronal backing material for such purpose comprises a non-opaque cellulose with a surface of light-tight paper 3 the type normally used for interleaving photographic When fabricating the laminate of intensifying screens from lead foil having a thickness in the order of 0.001 Ind], and a backing member, the two have been laminated together by nip rolling to eliminate wrinkles, blisters, and other surface irregularities that might later result in the absorption, the defraction, or the deflection of radiographic X-rays. The black facsimile paper may be nip rolled to the backing material or all three layers may be laminated simultaneously by nip rolling. An excellent intensifying screen 'has been prepared by this method from a laminate of a lead foil of about 0.001 inch thickness having tin on each flat surface thereof, together with a backing of material of cellulose acetate of about 0.003 inch thick, and a black facsimile paper.

My novel laminate intensifying screens are flexible, yet mechanically resistant to damage. The excitation potential of the special lead foil is in the most commercially useful range. The tin surface in the lead foil prevents corrosion and damage to the emsulsion of the adjacent film. Both of these lead and tin materials are commercially available with purities in excess of 99.9% and are free of contaminating radioactive materials or their decomposition products which are undesirable when used with photographic film of any kind.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

I claim.

1. An intensifying screen for use in radiography comprising (i) a thin flexible optically flat sheet of non-metallic material backing member, and

(ii) a sheet of lead foil having one of its surfaces in secured contact with said backing member and the other surface covered with a coating of tin, said lead alloy consisting essentially of between 2.7% and- 3.4% antimony, a total of between 0.075% and 3% tin, and the balance lead.

2. An intensifying screen as in claim 1, wherein the lead foil has both of its surfaces coated with tin.

3. An intensifying screen as in claim 2, wherein the lead foil is about 0.001 inch thick.

4. An intensifying screen as in claim 3, wherein the backing member is a flexible plastic sheet opaque to actinic light.

5. An intensifying screen as in claim 3, wherein the backing member is a flexible plastic having the surface opposite that secured in contact with the lead foil coated with a material opaque to actinic light.

6. An intensifying screen as in claim 3, wherein the backing member is a cast cellulose acetate.

7. An intensifying screen for use in industrial radiography comprising (i) a thin flexible optically flat sheet of non-metallic material backing member, and

(ii) a sheet of lead foil about 0.001 inch thick laminated continuously along one of its surfaces with said backing member,

(iii) and a coating of tin covering the opposite one of the surfaces of said sheet.

8. An intensifying screen as in claim 7, wherein the backing member is a plastic material opaque to actinic light.

9. An intensifying screen as in claim 7, wherein the backing member is a cellulose having one surface in contact with a flat surface of the lead foil, and a paper layer opaque to actinic light coating the opposite surface.

10. An intensifying screen as in claim 7, wherein said lead foil is an alloy consisting essentially of 3% antimony, a total of between 0.075% and 3% tin, and the balance of lead.

11. An intensifying screen for use in industrial radiography comprising (i) a thin flexible optically flat sheet of non-metallic backing material, and

(ii) on one surface of said backing material, a thin layer of lead having its outer surface fully coated with tin.

12. An intensifying screen as in claim 11, wherein the 5 backing material is a plastic opaque to actinic light and having its outer surface covered with a light-tight paper.

13. An intensifying screen as in claim 11, said backing material being opaque and essentially optically flat.

14-. An intensifying screen as in claim 11, said thin layer of lead and tin being about 0.001 inch thick.

15. An intensifying screen for use in radiography comprising (i) a thin flexible optically flat backing member,

(ii) a sheet of lead foil having one of its surfaces in secured contact with said backing member and the other surface covered with a coating of tin, said lead alloy consisting essentially of between 2.7 and 3.4% antimony, a total of between 0.075 and 3% tin, and the balance lead.

(iii) the lead foil being about 0.001 inch thick, and

(iv) said backing material being about 0.003 inch thick.

16. An intensifying screen for use in industrial radiography comprising (i) a thin flexible optically flat backing member, and

(ii) a sheet of lead foil about 0.001 inch thick laminated continuously along one of its surfaces with said backing member,

(iii) and a coating of tin covering the opposite one of the surfaces of said sheet,

(iv) the backing member being a flexible cellulose plastic sheet having one surface in contact with a flat surface of the lead foil, and a paper layer opaque to actinic light coating the opposite surface,

(v) said backing member being about 0.003 inch thick.

17. An intensifying screen for use in industrial radiography comprising (i) a thin flexible optically flat sheet of plastic backing material, and

(ii) on one surface of said backing material, a thin layer of lead having its outer surface fully coated with tin, (iii) said thin layer of lead and tin being about 0.001

inch thick, (iv) said backing material being about 0.003 inch thick.

References Cited by the Examiner UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner. 

1. AN INTENSIFYING SCREEN FOR USE IN RADIOGRAPHY COMPRISING (I) A THIN FLEXIBLE OPTICALLY FLAT SHEET OF NON-METALLIC MATERIAL BACKING MEMBER, AND (II) A SHEET OF LEAD FOIL HAVING ONE OF ITS SURFACES IN SECURED CONTACT WITH SAID BACKING MEMBER AND THE OTHER SURFACE COVERED WITH A COATING OF TIN, SAID LEAD ALLOY CONSISTING ESSENTIALLY OF BETWEEN 2.7% AND 3.4% ANTIMONY, A TOTAL OF BETWEEN 0.075% AND 3% TIN, AND THE BALANCE LEAD. 